1. Field of the Invention
The present invention relates to a thermosensitive recording material which utilizes a coloring reaction, for example, between an electron-donating coloring compound and an electron-accepting compound, more particularly to a transparent thermosensitive recording material which is suitable for a film for a video printer, and especially for an image formation sheet capable of producing a high quality black image therein, that is similar to a silver salt film designed for diagnostic and consulting purposes based on magnetic resonance imaging (MRI) and computed tomography (CT) in the medical field.
2. Discussion of Background
Various kinds of recording materials which employ the process of thermosensitive coloring reaction are proposed. Such a thermosensitive recording material is generally prepared by coating a mixture of a coloring agent (such as a dye) and a color developer on the surface of paper.
In recent years, disposal of the waste liquid caused by the wet-type image formation process for a silver salt X-ray film has become a serious problem in the medical field. Further, in line with the trend toward the formation of digital image, there is an increasing demand for a dry process using a transparent film capable of easily producing an image therein.
The dry process currently employed in the medical field is divided into the following three systems: (1) light-exposing and heat-fixing system, (2) thermal transfer system, and (3) direct thermosensitive recording system.
The thermosensitive recording material, which is used in the above-mentioned thermosensitive recording system, is usable as a recording material for an electronic computer, facsimile apparatus, ticket vending apparatus, label printer, and recorder. This is because the thermosensitive recording material has the advantages that complicated processes such as development and image fixing are not required, recording can be achieved for a short period of time using a relatively simple apparatus, there is no noise development, and the manufacturing cost is low.
In such a thermosensitive recording material, colorless or light-colored leuco dyes having a lactone, lactam, or spiropyran ring are used as coloring dyes, and organic acids and phenols are conventionally employed as color developers. There is also known a thermosensitive recording material of an organic silver salt type which employs a metallic salt of organic acid such as silver behenate as the coloring agent, and a reducing agent such as acid as the color developer. In addition, there is also known a reversible thermo-sensitive recording material which comprises the combination of a leuco dye and a color developer or the combination of a matrix resin and an organic low-molecular weight compound dispersed in the matrix resin. The reversible thermosensitive recording material is capable of reversibly forming an image therein and erasing the image therefrom by reversibly changing the transparency, the density, or the color of the recording material.
When the previously mentioned conventional thermosensitive recording material which comprises a leuco dye, a color developer, and a binder resin is exposed to strong light, for example, ultraviolet light, for a long period of time, there gradually appear unfavorable phenomena such as yellowing of a background portion (non-image area) of the recording material, and decreasing of the image density of an image portion formed on the recording material unless any additive is employed. In other words, there occurs the problem that the image recognition gradually becomes difficult as the image bearing thermosensitive recording material is exposed to light. This problem is produced likewise in other thermosensitive recording systems.
To solve the above-mentioned problem, it is proposed to add an additive for promoting the light resistance of the recording material, such as an ultraviolet light absorber, which additive will be hereinafter referred to as a light resistant additive, to any layers that constitute the thermosensitive recording material. The above-mentioned light resistant additive has been conventionally studied, and there are proposed as the light resistant additives a benzotriazole ultraviolet light absorber (Japanese Laid-Open Patent Application 61-193883), a fluorescent whitening agent (Japanese Laid-Open Patent Application 62-184880), a hindered amine light stabilizer (Japanese Laid-Open Patent Application 63-137887), finely-divided particles of inorganic oxides (Japanese Laid-Open Patent Application 7-25147); and a mixture of the above-mentioned materials (Japanese Laid-Open Patent Application 8-282114). When the amount of light resistant additive is increased in the transparent thermosensitive recording material, the color change of the recording material caused by light exposure can be inhibited more effectively.
However, a transparent thermosensitive recording material has the drawback that the change in color or density of the image portion is striking when compared with the reflection type thermosensitive recording material. As is apparent from the light absorbance curve of a transparent thermosensitive recording material, the absorbance substantially increases in proportion to the rise in the density, and the absorption peak is very sharp when the light absorbance curve of a transparent thermosensitive recording material is compared with that of a conventional reflection type thermosensitive recording material. Further, with respect to the transparent thermosensitive recording material, the color development sensitivity and the color tone of a produced image may considerably vary depending upon the amount of light resistant additive added to the recording material. Therefore, the target to be attained by the transparent thermosensitive recording material becomes higher than that by the reflection type thermosensitive recording material. In other words, the control of the above-mentioned change becomes considerably difficult in the transparent thermosensitive recording material
The light resistant additive shows slight absorption in the visible wave range, so that the color of the background portion (non-image area) of the conventional transparent thermosensitive recording material appears different depending on the employed light source. This tendency is called a light-source dependence in the present invention.
The light-source dependence will now be explained in detail using two kinds of light sources, that is, a diffused light as represented by d/0, and a specular light as represented by 0/0. In the above, d/0 and 0/0 are geometrical conditions of a lighting and a light receptor in measurement of the color of a non-image area of the transparent thermosensitive recording material. More specifically, "d" denotes diffused directions, and "0" denotes the angle of 0.degree.. The geometrical condition of d/0 means that the diffused light enters a transparent recording material, and the light transmitted by the transparent recording material enters the light receptor in a vertical direction. On the other hand, the geometrical condition of 0/0 means that the specular light enters a transparent recording material, and the light transmitted by the transparent recording material enters the light receptor in a vertical direction.
When the specular light (0/0) is used as the light source, the absorbance of a transparent thermosensitive recording material detected by the light receptor is higher as a whole, in particular, in the short wavelength region, than the absorbance thereof obtained by using a diffused light (d/0) as the light source.
Even though the transparent thermosensitive recording material is entirely colored, the observers perceive the difference in color depending upon the light source. In the case where the color of the background is a complementary color of the perceived color when either of the above-mentioned light sources is employed, the light-source dependence becomes more noticeable.
In Japanese Laid-Open Patent Application 4-197778, there is proposed a transparent thermosensitive recording material which comprises a light shielding layer capable of showing a transmittance of 5% or less when irradiated with light of 370 nm, a transmittance of 70% or less when irradiated with light of 400 nm, and a transmittance of 70% or more in the entire visible light range. When the above mentioned specular light (0/0) is used as the light source, the light-source dependence of a background portion (non-image area) of the recording material becomes considerably large due to the presence of the above-mentioned light shielding layer.
The absorption in the near ultraviolet region is influenced by the haze of the transparent thermosensitive recording material. In general, when the haze of the transparent recording material is high, light diffusion takes place at many places in the layers of the recording material. Such a transparent recording material having a high haze value does not easily transmit the lights in the short wavelength region. The result is that the dependence of the color tone of the background portion of the recording material on the employed light source is increased, and the transparency is lowered, thereby making the image recognition more difficult.
The image recognition performance is further influenced by other factors, such as fogging of the background of the recording material, difference in refractive index of binder resins employed in the layers, and the particle size of a filler dispersed in the recording material. The control of the above-mentioned factors becomes important for the improvement of image recognition performance.
To reduce the light-source dependence, it is proposed to increase the amount of a dye or pigment used as the colorant when the transparent thermosensitive recording material is entirely colored. However, this method is attended by the problem such as the increase of the background density.